The present invention relates to protective membranes for providing protection against corrosion and/or water penetration. The protective membrane of the present invention is capable of adhering to cement mortar or concrete that sets and hardens whilst in contact with it. It is therefore particularly useful for providing such protection to buildings and concrete structures and also to metal pipes and is therefore described below with respect to such applications, but it will be appreciated that the invention could advantageously be used in other applications, e.g., for protecting sheet metal constructions.
The main cause of degradation of reinforced concrete structures is corrosion of the steel reinforcement. Initially, the steel is protected from corrosion by the "passivation effect" of the high alkalinity of the concrete. However, in the course of time, the penetration of carbon dioxide from the air into the concrete converts the highly alkaline calcium hydroxide, present in the concrete, into non-alkaline calcium carbonate, thus diminishing the alkalinity of the concrete and the above passivation effect. In addition, the penetration into the concrete of chlorides dissolved in water also diminishes the passivation effect. With the diminishing or elimination of the passivation effect, oxygen in the presence of moisture in the air initiates corrosion of the steel reinforcement at a rate dependent on the rate of supply of the oxygen.
Steel pipes are commonly protected against corrosion by bonding to the pipe polyethylene or other polymeric tape wrapped around it or by forming a layer of cementitious mortar around the pipe. However, polymeric tape protection for steel pipes does not provide the above-described passivation effect to the pipe to inhibit corrosion; moreover, any imperfections in the polymeric tape which may be initially present, or which may appear during use, expose the pipe to corrosion in the vicinity of the imperfection. Whilst a cementitious mortar layer applied around the pipe provides the above-described passivation effect to the pipe surface, this effect is diminished during the course of time as the cementitious layer is unprotected and therefore undergoes carbonation; moreover, the penetration of salts in a soluble form through the unprotected concrete also diminishes this passivation effect.
Another method of protecting a steel pipe is to apply a concrete layer around the pipe, and to simultaneously wrap a polymeric tape over the concrete layer. However, concrete does not adhere well or at all to polymeric surfaces, and therefore the effectiveness of the polymeric tape in preventing carbonation and the penetration of moisture and salts is limited.
Coatings of bituminous materials or polymeric resins are also frequently used to waterproof concrete structures and steel pipes and to protect them against corrosion. However, the application of such materials over a cementitious surface prevents the possibility of bonding an additional layer of concrete or concrete paste mortar over that coating after it has hardened since the hydration products of cement mortar will generally not bond well to such coatings.
A textile fabric bandage, pre-impregnated with a cementitious mix is currently used in building practice. The impregnated bandage is a carrier of dry "dormant" cementitious glue. On wetting, the dry cementitious mass attains a pasty consistency and the bandage is capable of adhering to the hydration products of cementitious materials. If cement mortar is cast over the pre-impregnated bandage, on hardening it will adhere to the bandage and the bandage will form a skin over the cementitious layer. If cement mortar is cast over both sides of the pre-impregnated bandage, on hardening a multilayered solid mass is attained with interlayer continuity. The bandage is used for waterproofing and repair.